There has been known a technique for improving the characteristics of polishing material particles by providing a cover layer on the surface of each polishing material particle. For example, Patent Literatures 1 and 2 propose polishing material particles having a core-shell structure and having an average particle size ranging from 5 to 300 nm. The shell part of this core-shell structure has a thickness ranging from 1 to 50 nm, and is constituted by a silica-based composite oxide including silica as a main component. The core part is made of MnO, SiO2, Al2O3, ZrO2, SnO2, ZnO, CeO2, or TiO2. The technique disclosed in these literatures has the aim of preventing alkali metal from remaining on the polished substrate by trapping, with the shell part, the alkali metal included in the core part, and the aim of adjusting polishing characteristics by providing the shell part or by changing the components of the shell part.